Recently, apparatus and methods have been developed with the ability to generate an image on media using a low power laser. Such lasers are employed to read and write data on the data side of optical storage discs, such as CDs, DVDs and the like. Typically, various types of data are written on a data side of a disc by a laser beam while the disc is rotating. Data may be recorded by making marks on the recording media, indicative of a zero or one. Various data writing strategies have been employed, for the purposes of creating desirably shaped marks precisely positioned and having sharp edges to enable detection.
The side of a data disc opposite the data side is often used for handwriting or affixing or marking a label with descriptions and illustrations corresponding to the recorded data. Recently, apparatus and methods have been developed with the ability to generate a label on the non-data side of an optical disc using the same laser that was employed to read and write digital or electronic data on the data side of the disc. See U.S. Patent Application Publication No. 2003/0108708 (Anderson, et al.), disclosing the use of laser sensitive materials on a disc label that react chemically with the application of light and heat and result in changes in color and shading on the label.
In making marks on either side of a disc using an optical disc drive, the laser power output may be reduced with time, through dust accumulation or contamination build-up on optics associated with the laser. Effective laser power output may also be diminished over time due to attrition of laser output or front end sensors because of aging. Consequently, initial laser power output that was calibrated and set in the factory with the front sense diode voltage may be reduced significantly during usage. Some attrition of laser power output is not sensed by the front sense diode, since it measures power leaving the laser prior to the laser beam being processed by the optics. In addition, the outputs from photo-detectors used for sensing the reflections of laser power output may be skewed or distorted due to temperature variations.
In marking on the label side of the disc, using the technology described in the Anderson application, the chemistry on the label is optimized to absorb as much light as possible at the wavelength of the writing laser beam. As a result, there is very little reflectivity of light at the frequency of the writing laser, and the change in reflectivity between written and unwritten areas may be less than the noise level of the detection circuit. Accordingly, it is difficult to calibrate optical power on the label recording medium, because of the low contrast between unwritten and written medium.
One current method for calibrating laser power output is to measure the power required to correctly mark a medium using a marking procedure for the medium. Then the power measurement method is replicated in the drive factory, and each drive is tuned to the power specified by the medium for good marking. This method relies on having effective correlation between these two power measurements, which is sometimes not the case. Moreover, this approach does not involve measuring marks made in the field to determine whether effective optical density is being achieved.